As wireless communication systems evolve, wireless system design has become increasingly demanding in relation to equipment and performance requirements. Future wireless systems, which will be third generation (3G) and fourth generation (4G) systems compared to the first generation (1G) analog and second generation (2G) digital systems currently in use, will be required to provide high quality high transmission rate data services in addition to high quality voice services. Concurrent with the system service performance requirements will be equipment design constraints, which will strongly impact the design of mobile terminals. The 3G and 4G wireless mobile terminals will be required to be smaller, lighter, more power-efficient units that are also capable of providing the sophisticated voice and data services required of these future wireless systems.
Time-varying multi-path fading is an effect in wireless systems whereby a transmitted signal propagates along multiple paths to a receiver causing fading of the received signal due to the constructive and destructive summing of the signals at the receiver. Several methods are known for overcoming the effects of multi-path fading, such as time interleaving with error correction coding, implementing frequency diversity by utilizing spread spectrum techniques, or transmitter power control techniques. Each of these techniques, however, has drawbacks in regard to use for 3G and 4G wireless systems. Time interleaving may introduce unnecessary delay, spread spectrum techniques may require large bandwidth allocation to overcome a large coherence bandwidth, and power control techniques may require higher transmitter power than is desirable for sophisticated receiver-to-transmitter feedback techniques that increase mobile terminal complexity. All of these drawbacks have negative impact on achieving the desired characteristics for third and fourth generation mobile terminals.
Antenna diversity is another technique for overcoming the effects of multi-path fading in wireless systems. In transmit diversity, a signal is multiplexed and processed to generate a number of separate signals that are then transmitted via two or more physically separated antennas. Similarly, in reception diversity, two or more physically separated antennas are used to receive a signal, which is then processed through combining and switching to generate a received signal. Various systems, known as multiple-input multiple-output (MIMO) systems, employ both transmit diversity and reception diversity, and provide multiplexing and diversity gains in wireless communication.
One transmit diversity technique by which two transmit antennas redundantly send information to a single receiving antenna is disclosed in U.S. Pat. No. 6,185,258, entitled: Transmitter Diversity Technique for Wireless Communications, issued Feb. 6, 2001 to Alamouti et al., the contents of which is incorporated herein by reference. In accordance with the Alamouti transmit diversity technique, information is transmitted temporally during “time slots,” the duration of which is small enough so that the transmission quality on each of the two channels is effectively constant during the time slot. A time slot is divided into symbol periods, each symbol period representing the time in which a single symbol is transmitted from an antenna.
In accordance with the Alamouti transmit diversity technique, in a time slot with a duration of two symbol periods, a first antenna transmits a symbol z1 during the first symbol period and a symbol −z2* during the second symbol period, and a second antenna transmits a symbol z2 during the first symbol period and a symbol z1* during the second symbol period. Here, “a*” denotes the complex conjugate of “a” (i.e., if a=x+yj, then a*=x−yj). Time slots can be referred to as “time-space slots” in recognition that more than one antenna is transmitting—emphasizing that there is space diversity—or can simply be called “slots.” The Alamouti matrix CAla is shown below, with each row corresponding to a transmit antenna and each column corresponding to a symbol period.
            C      A1a        ⁡          (                        z          1                ,                  z          2                    )        =      [                                        z            1                                                -                          z              2              *                                                                        z            2                                                z            1            *                                ]  
If one of the two antennas is transmitting more robustly than the other during the time slot, both symbols can be derived solely from the stronger of the two transmissions.
During the third and fourth symbol periods, a new slot is formed in which Z3 assumes the role of z1 and z4 assumes the role of Z2, and so on for subsequent time slots and respective symbol periods. Therefore, the transmit antennas transmit according to a sequence of 2×2 Alamouti codes. The kind of matrix, such as the 2×2 Alamouti matrix, that is used to represent transmit diversity over symbol periods is called a “space-time block code.”Here, the space-time block code and the time slot happen to coincide although this is not always the case. The diversity here is two or “two-fold,” because each symbol is transmitted twice, by virtue of a delayed identical copy or delayed complex conjugate (or the negative of the complex conjugate or “negative complex conjugate”). Under the assumption that a single transmitter transmits one symbol per symbol period, the number of symbols that are transmitted per symbol period in a communication system is known as the “symbol rate.” The symbol rate here is one since a symbol is considered to be the same, for this purpose, as its complex conjugate or negative complex conjugate.
In contrast to space-time coding techniques, such as the Alamouti transmit diversity technique, space-frequency coding techniques rely on coding across space and frequency by dividing the symbol stream into several parallel symbol streams and modulating each of these streams onto separate carriers or subcarriers at separate frequencies or within separate frequency bins. Further coding techniques, referred to as space-time-frequency coding techniques, offer a combination of space-time coding and space-frequency coding by coding symbols among transmit antennas in time and frequency. And although conventional space-time, space-frequency and space-time-frequency coding techniques are adequate to overcome at least some of the effects of multi-path fading in wireless systems, it is typically desirable to improve such techniques.